Screw extruders which comprise a screw device mounted foe rotation in, and surrounded by a barrel are well known. Such extruders have a feed aperture for the material to be treated, an outlet for the treated material downstream of such feed aperture, a drive unit for the screw located upstream of the feed aperture, and reverse degassing arrangement which is disposed between the feed aperture and the drive unit.
The material to be treated, such as a melt produced during a polymerisation process, is usually supplied to such an extrusion device from, for example, the reactor in which the polymerisation is effected, at a temperature in excess of 150.degree. C. The melt is entrained by the flights of the screw device and is forced to travel along a prescribed path in the conveying direction of the screw extruder. Gases contained therein and small portions of melt such as oligomers can escape from the melt. This is due to the fact that a gap is formed between the screw and the barrel or, if sealing elements are provided in the barrel, around the sealing elements. These gaseous and small solid portions tend to travel in a direction opposite the main flows of the melt, that is to say, towards the drive unit.
In many cases, the extrusion screw device is provided with a return winding at the end of the extrusion housing. This prevents excessively large quantities of the polymerised substances from passing into the drive gear.
An extruder incorporating a degassing arrangement is disclosed in German Patent Specification No. DE 3 615 609 C2. The degassing arrangement disclosed therein is formed from a stuffer-box housing, which covers and seals the screw shaft and is connected to the barrel of the extruder. The end of the internal wall of the housing which faces the barrel is provided with a helical groove, which groove extends in the opposite direction to the screw flight. In communication therewith is an accentric annular chamber, from which one or a plurality of apertures extend. However, the disadvantage of this known degassing arrangement is that it cannot be universally used for any melt. Extrusion screws are used, which have a return flight at the inlet to the degassing arrangement. The degassed material has a relatively long dwell-time in the region of the return flight and, when the product is returned from the degassing region into the main product stream, material is deposited. This is particularly disadvantageous if thermally unstable products are being treated.
An additional disadvantage of such a known degassing arrangement is that the reverse degassing chamber is easily blocked if products having a high melt-flow index and/or copolymerised substances are being treated. These have the disadvantageous properly of adhering to metals, and the efficiency of the reverse degassing is adversely affected by such adhesion.